The influence of nitrous oxide to supplement fentanyl/low-dose propofol anesthesia on transcranial myogenic motor-evoked potentials during thoracic aortic surgery

Abstract

Objective: Intraoperative monitoring of myogenic motor evoked potentials to transcranial electrical stimulation (tc MEPs) is a new method to assess the integrity of the motor pathways. The authors studied the effects of 50% nitrous oxide (N 20) and a low-dose propofol infusion on tc MEPs paired electrical stimulation during fentanyl anesthesia with partial neuromuscular blockade. Design: Cross-over study. Setting: St Antonius Hospital, Nieuwegein, The Netherlands. Participants: Ten patients scheduled to undergo surgery on the thoracoabdominal aorta were studied; 6 women aged 54 to 69 years and 4 men aged 68 to 77 years. Interventions: After achieving a stable anesthetic state and before surgery, tc MEPs were recorded during four 15-minute periods: (I) air/oxygen (O 2; F IO 2 = 50%); propofol target blood concentration, 0.5 μg/mL; (II) N 2O/O 2 (F IO 2 = 50%); propofol target blood concentration, 0.5 μg/ mL; (III) N 2O/O 2 (F IO 2 = 50%; propofol target blood concentration, 1.0 μg/mL; and (IV) air/O 2 (F IO 2 = 50%); propofol target blood concentration, 1.0 μg/mL. Measurements and Main Results: Tc MEPs were recorded from the right extensor digitorum communis muscle and the right tibialis anterior muscle. The right thenar muscle was used for recording the level of relaxation; the T1 response was maintained at 40% to 70% of the control compound muscle action potential. There was no significant difference in onset latency among the four phases. The addition of N 2O and doubling the target propofol infusion to 1.0 μg/mL resulted in a 40% to 50% reduction of tc MEP amplitude recorded in the extensor digitorum communis muscle and tibialis anterior muscle ( p < 0.01). During each phase, tc MEPs could be elicited and interpreted, except in one patient, in whom no tc MEPs could be elicited in the leg because of technical problems. Conclusion: The data indicate that tc MEP monitoring is feasible during low-dose propofol, fentanyl/50% N 2O in O 2 anesthesia and partial neuromuscular blockade.